The present invention relates to a control system for an active, adaptive vibration and noise attenuation system. The present invention serves as the intelligence of an overall system that has several parts. Generally, the other parts of the control system are sensors for measuring the objectionable vibration and noise and one or more controlled devices for providing a mechanism for altering the production of noise and vibration. In particular, the present invention relates to a control system combining the results of multiple paths to generate a resulting vibration and noise control signal with at least one attenuation path used to generate vibration and noise attenuation signals and at least one other path used to generate signals which seek to control the position of the altering mechanism to prevent saturation of the mechanism.
The present invention also relates generally to a system for controlling an active system for reducing the transmission of vibration and noise passing from a vibrating component to a structure and, more particularly, to a system for controlling an active vibration and noise reduction system for use on a rotary wing aircraft.
Even more particularly, the present invention comprises an Active Transmission Mount Controller (ATM Controller) to be used to control a number of hydraulic actuation systems utilized in active cancellation of vibration in rotary wing aircraft. The ATM Controller controls hydraulic actuators located in-line between each transmission foot and the airframe. The ATM Controller produces outputs that are based on the fundamental blade rotational rate as well as multiples of this rate. In addition, the ATM Controller produces a position control signal to maintain the relative position of the transmission foot and the aircraft.
Significant effort has been devoted to reducing the vibratory and acoustic loads on aircraft, particularly rotary wing aircraft such as helicopters, and the resulting vibration and noise that develops within the aircraft. A primary source of vibratory and acoustic loads in a helicopter is the main rotor system.
The main rotor system of a helicopter includes rotor blades mounted on a vertical shaft that projects from a transmission, often referred to as a gearbox. The gearbox comprises a number of gears which reduce the rotational speed of the helicopter""s engine to the much slower rotational speed of the main rotor blades. The gearbox has a plurality of mounting xe2x80x9cfeetxe2x80x9d which are connected directly to structure in the airframe which supports the gearbox.
The main rotor lift and driving torque produce reaction forces and moments on the gearbox. All of the lift and maneuvering loads are passed from the main rotor blades to the airframe through the mechanical connection between the gearbox feet and the airframe. The airframe structure which supports the gearbox is designed to react to these primary flight loads and safely and efficiently transmit the flight loads to the airframe.
In addition to the nearly static primary flight loads, the aircraft is also subjected to vibratory loads originating from the main rotor blades and acoustic loads generated by clashing of the main rotor transmission gears. The vibratory loads are strongest at a frequency equal to the rotational speed of the main rotor blades (P), which is generally between about 4 and about 5 Hz, multiplied by the number of rotor blades, typically 2 or 4. The product of the main rotor blades rotational speed and the number of blades is called the xe2x80x9cfundamentalxe2x80x9d. Tonals of decreasing vibratory strength occur at multiples of two, three and sometimes four of the fundamental. For example, for a 4 bladed rotor, this would correspond to 8P, 12P, and 16P.
The acoustic loads generated by the transmission gears are at a frequency that the gear teeth mesh with and contact each other, and are thus related to the type of construction and gear ratios used in the transmission. The acoustic loads also include a fundamental and tonals of decreasing strength at integer multiples of the fundamental. Typically, the noise generated by gear clashing is in the range of about 500 Hz to about 3 kHz.
The vibratory and acoustic loads produce vibrations and audible noise that are communicated directly to the helicopter airframe via the mechanical connection between the gearbox and the airframe. This mechanical connection becomes the xe2x80x9centry pointxe2x80x9d for the unwanted vibration and noise energy into the helicopter cabin. The vibrations and noise within the aircraft cabin cause discomfort to the passengers and crew. In addition, low frequency rotor vibrations are a primary cause of maintenance problems in helicopters.
In the past, xe2x80x9cpassivexe2x80x9d solutions have been tried for reducing the vibratory and acoustic loads on aircraft and the resulting vibration and noise that develops within the aircraft. For noise reduction, passive systems have employed broadband devices such as absorbing blankets or rubber mounts. However, broadband passive systems have generally proven to be heavy and, consequently, not structurally efficient for aircraft applications where weight is paramount. Additionally, broadband passive systems are not very effective at reducing low frequency vibration. A passive technique for reducing vibration involves the installation of narrowband, low frequency vibration absorbers around the aircraft that are tuned to the vibration frequency of interest, typically the fundamental. These narrowband, passive vibration reduction systems are effective, but the number of vibration tonals present in a helicopter may require a number of these systems which then adds significant weight. Additionally, narrowband passive systems work best when placed at ideal locations about the helicopter airframe, many of which may be occupied by other equipment.
More recently, xe2x80x9cactivexe2x80x9d vibration and noise reduction solutions are being employed since active systems have a much lower weight penalty and can be effective against both low frequency vibration and higher frequency noise. Active systems utilize sensors to monitor the status of the aircraft, or the vibration producing component, and a computer-based controller to command countermeasures to reduce the vibration and noise. The sensors are located throughout the aircraft and provide signals to the adaptive controller. The controller provides signals to a plurality of actuators that are located at strategic places within the aircraft. The actuators produce controlled forces or displacements which attempt to minimize vibration and noise at the sensed locations.
Low frequency motion (i.e., vibration) behaves according to rigid body rules and structural models can be used to accurately predict the nature and magnitude of the motion. Since low frequency motion is easily modeled, its negative effects can be cancelled with an active system of moderate complexity. High frequency motion (i.e., noise) at the transmission gear clash frequencies does not obey the rigid body rules present at low vibration frequencies. The use of riveted airframes in combination with the complex mode shapes present at high frequencies makes structural models much less accurate. As a result, active systems for high frequency energy reduction become more complex, requiring large numbers of actuators and sensors to counter the more complex modal behavior of the airframe structure.
Some active systems utilize hydraulic actuation systems and hydraulic actuators to reduce vibration and noise. The hydraulic actuation system is preferred since the hydraulic system provides the necessary control bandwidth and authority to accommodate the frequencies and high loads typically experienced in an aircraft such as a helicopter. Additionally, aircraft typically have hydraulic power sources with spare capacity which can be utilized or augmented.
Two methods of actuator placement are frequently used in active systems: (1) distribute the actuators over the airframe, or (2) co-locate the actuators at, or near, the vibration or noise entry point. The co-location approach places the actuators at or near the structural interface between the transmission and airframe stopping vibration and noise near the entry point before it is able to spread out into the aircraft. This has the advantage of reducing the number of actuators and the complexity of the control system. Active systems using this approach employ actuators mounted in parallel or in series with the entry point to counteract the vibration and noise.
The distributed actuator approach requires a large number of actuators for controlling noise due to the high frequencies, and their associated short spatial wavelength. The large number of actuators can drive up weight and add significantly to control system complexity. One distributed actuator active noise reduction system for use in a helicopter application uses more than 20 actuators to control transmission noise. Distributed actuators for low frequency vibration will be less numerous and are effective at reducing vibration at the sensor locations, but can drive vibration at other areas of the aircraft to levels exceeding those already present.
The parallel actuator approach is effective for low frequency vibration but can produce counteracting forces in the supporting structural elements which can exceed the design limit of the elements and lead to premature failure. Additionally, the parallel approach is not effective at reducing noise because the parallel actuators provide a direct path for noise entry.
The series approach is the most effective in reducing cabin vibration and avoids the introduction of unwanted vibrations. This approach uses actuators mounted in series between the transmission gearbox feet and airframe support structure. In this approach, the gearbox and airframe are isolated from each other connected only by actuators. The gearbox vibrates in its own inertial frame separately from the airframe""s inertial frame, isolating the gearbox and airframe in a dynamic sense. This approach interrupts the transmission of vibratory and acoustic energy through the principal entry point thereby preventing vibration and noise from entering the airframe. For this approach to be effective, the vibration and noise isolation system must support the large, static primary flight loads along an axis also requiring dynamic isolation. This system must also maintain the average static position of the transmission relative to the airframe for proper operation of the other helicopter systems, particularly the helicopter engines that couple into the transmission. However, in the series approach, the high frequencies associated with noise lead to complex motions at the entry point which, if fully addressed, may lead to large and heavy actuators to actively control all degrees of freedom at each entry point.
A more efficient way for reducing both vibration and noise in aircraft applications, and particularly helicopters, combines an active system for low frequency vibration reduction with a passive system for high frequency noise reduction. Preferably, the active vibration reduction system will isolate the vibratory load source, such as the main rotor system of the helicopter, and prevent the low frequency vibration generated by the main rotor system from being transmitted to the airframe. The system should efficiently pass the primary flight loads while maintaining the average static position of the gearbox relative to the airframe.
Adaptive controllers for active vibration reduction systems are well known in the art. These controllers monitor vibrations and seek to generate signals which drive devices producing canceling vibrations. The controlled devices used to cancel vibrations act either upon the body producing the objectionable vibrations or the controlled devices may act upon some connection point between the vibration generating machinery and the vibration measurement point. Such connection point efforts include actuators which connect helicopter transmission feet to helicopter cabins.
One method known in the art is to measure the noise and vibration disturbances at locations where cancellation is desired and to feedback this information into an active controller which then makes alteration/cancellation adjustments to reduce the noise and vibration disturbances. Feedback systems tend to be effective when the time delay through the controller actuator and sensors is kept to a minimum.
Existing adaptive controllers assume sufficient authority exists in the vibration cancellation mechanism to respond to the vibration cancellation signals. This may not always be true. For example, a hydraulic actuator used to produce cancellation vibrations may reach the maximum extent of actuation. In such a situation, the actuator could not continue to respond to cancellation signals until the actuator moves sufficiently away from a maximum actuation extent. Cessation of ability to respond has at least two drawbacks. The first is an obvious reduction in the cancellation of the vibration being controlled by the impaired cancellation mechanism. The second drawback is that a mechanism such as an actuator at full extent may exhibit characteristics similar to a fixed mount. Such a fixed mount might reduce the effectiveness of passive vibration reduction techniques used in conjunction with the active vibration control system.
For the foregoing reasons, there is a need for a new control system for active reduction of both vibration and noise. The new controller will transmit output vibration cancellation signals which control an active vibration cancellation mechanism. Such vibration cancellation mechanism will be located within the connection points and in series between a vibration generating component and the mounting location of the component. The controller should employ two or more control paths to ensure that the vibration cancellation mechanism maintains the relative position between the vibration generating component and the mounting location and has sufficient authority to respond to the transmitted vibration cancellation signals.
It is an object of the present invention to provide a controller for an active control system for simultaneously reducing both vibration and noise in aircraft applications, and particularly helicopters.
Another object of the present invention is to provide a controller for an active device and system for isolating the main rotor system of a helicopter from the airframe for preventing the low frequency vibration generated by the main rotor system from being transmitted to the airframe.
A further object of the present invention is to provide a controller for an active vibration reduction system for passing the primary flight loads of the helicopter from the main rotor system to the airframe while maintaining the average static position of the gearbox relative to the airframe.
According to the present invention, a control system is provided for reducing vibration generated by a vibrating plant, the vibrating plant including a vibrating component, a structure and a mount for mounting the vibrating component to the structure. The control system comprises means for producing controlled vibrations within the mount. Sensors are provided for sensing the current position of the controlled vibration producing means, the vibration being transmitted through the mount from the vibrating component to the structure, and at least one of the characteristic frequencies at which the vibrating plant operates and developing signals indicative thereof. A first controller receives as input the signal from the position sensor located on the controlled vibration producing means and generates an output signal. A second controller receives as input the transmitted vibration sensor signal and the plant rotational sensor signal and generates an output signal. Means are provided for combining the output signals from the first and second controllers into a control signal for controlling the vibration producing means such that the vibration transmitted from the vibrating component to the structure through the mount is reduced.
Further according to the present invention, a control system is provided for an active system for reducing vibration generated by a vibrating plant, the vibrating plant including a vibrating component, a structure and a hydraulic mount for mounting the vibrating component to the structure. The control system comprises at least one hydraulic actuator for producing controlled vibrations within the mount. Sensors are provided for sensing the current position the hydraulic actuator relative to the mount, vibrations being transmitted from the vibrating component through the hydraulic mount to the structure, and at least one of the characteristic frequencies at which said vibrating plant operates. The sensors produce signals representative thereof. A fixed, low bandwidth, near-DC, proportional/integral/derivative (PID)-based broadband control compensation feedback position controller utilizes the position sensor signal to produce position control signals to minimize the offset between the sensed hydraulic actuator position and a predetermined hydraulic actuator position. An adaptive Filtered-X least-mean-square (LMS) based narrow-band vibration controller utilizes the vibration sensor signal to produce vibration control signals at multiple frequencies of the sensed plant characteristic frequencies. Means are provided for combining the position control signals with the vibration control signals and generating an output signal which the hydraulic actuator responds to for producing controlled vibrations in the mount for reducing vibrations transmitted through the mount from the vibrating component to the structure.
Also according to the present invention, a control system is provided for active vibration reduction in a rotary wing aircraft including an airframe and a main rotor system having an engine, a rotor and a transmission gearbox mounted to the cabin support beam located at the top of the airframe by at least one hydraulic mount. The gearbox converts the engine force into the rotational force of a rotorshaft. The control system comprises at least one hydraulic actuator for producing controlled vibrations within the mount. Sensors are provided for sensing the current position of the hydraulic actuator, vibrations being transmitted from the main rotor system through the mount to the airframe, and for sensing the rotational frequency of the rotorshaft and producing signals representative thereof. A fixed, low bandwidth, near-DC, broadband control compensation feedback position controller utilizes the actuator position sensor signal to produce quasi-static position control signals to minimize the offset between the sensed hydraulic actuator position and a predetermined hydraulic actuator position. An adaptive Filtered-X LMS based narrow-band vibration controller utilizes the vibration sensor signal to produce vibration control signals. Means are provided for combining the position control signals with the vibration control signals and generating an output signal which the hydraulic actuator responds for producing controlled vibrations in the mount for reducing vibrations transmitted through the mount from the main rotor system to the airframe.
A feature of the actuator position controller is the attenuation output signals are maintained within a maximum range to which the vibration producing means is capable of responding. The extent of actuation of the actuator is thus maintained around a predetermined point, preferably a center point, to ensure the actuator has sufficient authority to respond to the vibration cancellation signals. The position controller includes a scaling function, a band elimination function, an objective function and a compensation function. In one embodiment, the compensation function produces the position control signal utilizing proportional, integral, derivative control compensation.
The vibration controller features a frequency filter adaptive to isolate sensed vibration signals at frequencies which are multiples of the sensed characteristic or rotorshaft rotation frequency, an objective function characterizing the magnitude of the isolated signals, a compensation function producing a correlation between the isolated signals and the control signal for the controlled vibration producing means, and an adaptive filter which generates attenuation output signals minimizing the isolated, correlated signals. In one embodiment, the frequency filter comprises a band-pass filter and a notch filter receiving as input the characteristic or rotorshaft rotation frequency, the notch filter adapting its filter window based on the input frequency. The vibration controller also features a frequency downshift function which converts the vibration sensor signals to signals at baseband DC and a frequency upshift function which converts the baseband DC signals into in-band, attenuation path-based, vibration control signals. The vibration controller may also include an input function which performs antialiasing and scaling functions on the vibration sensor signals, a normalization function which normalizes the isolated signals, an output function which scales the vibration control signals, and a weight limiting function which evaluates the vibration control signals and transmits a freeze signal to the adaptive filter function affecting the adaptive abilities of the adaptive filter function.